Because somebody who's coming 1st in all their classes a school ranked 500th isn't necessarily going to get 99.95 purely because their first. I went to a low ranked school and there were a few people who came first in all their subjects, but the cohort was very spread in their marks.
Coming first at a school that consistently performs well, ie. James Ruse, is more likely to help you because your entire grade will help bring you up.